1. Field of the Invention
The present invention relates to golf equipment comprising a fast-chemical-reaction-produced component. The component is made from a reaction injection molding process in which the stoichiometry between reactants is imbalanced.
2. Description of the Related Art
Reaction injection molding (“RIM”) is a process used to make golf equipment or products such as golf balls and/or components thereof, including covers, cores, inner layers, etc. Highly reactive liquids are injected into a closed mold, mixed usually by impingement and/or mechanical mixing in an in-line device such as a “peanut mixer”, and polymerized primarily in the mold to form a coherent, one-piece molded article. When used to make a polyurethane or polyurea article, RIM usually involves a rapid reaction between two types of reactants: (a) a polyol or other material with an active hydrogen, such as a polyfunctional alcohol or amine (hereinafter referred to as “polyol”); and (b) an isocyanate-containing compound (hereinafter referred to as “isocyanate”). The reactants are stored in separate tanks prior to molding and may be first mixed in a mix-head upstream of a mold and then injected into the mold. The liquid streams are metered in the desired weight to weight ratio and fed into an impingement mix-head, with mixing occurring under high pressure, e.g., 1500 to 3000 psi. The liquid streams impinge upon each other in the mixing chamber of the mix-head and the mixture is injected into the mold. One of the liquid streams typically contains a catalyst for the reaction. The reactants react rapidly after mixing to gel and form polyurethane or polyurea polymers. Such mixtures, typically reacting in five minutes or less, are herein referred to as “fast-chemical-reaction” mixtures.
RIM offers several advantages over conventional injection and compression molding techniques. For example, the reactants are simultaneously mixed and injected into the mold, forming the desired component. In conventional techniques, the reactants must first be mixed in a mixer separate from the molding apparatus, and then added into the apparatus. In such a process, the mixed reactants first solidify and must later be melted in order to properly mold the desired component.
Additionally, the RIM process requires lower temperatures and pressures during molding than injection or compression molding. Under the RIM process, the molding temperature is maintained from about 90 to about 180° F., and usually at about 100-160° F., in order to ensure proper injection viscosity. Compression molding is typically completed at a higher molding temperature of about 320° F. (160° C.). Injection molding is completed at an even higher temperature range of 392-482° F. (200-250° C.). Molding at a lower temperature is beneficial when, for example, the cover is molded over a very soft core so that the very soft core does not melt or decompose during the molding process.
Moreover, the RIM process creates more favorable durability properties in a golf ball component than conventional techniques. For example, a golf ball cover produced by a RIM process has a uniform or “seamless” cover in which the properties of the cover material in the region along the parting line are generally the same as the properties of the cover material at other locations on the cover, including at the poles. The improvement in durability is due to the fact that the reaction mixture is distributed uniformly into a closed mold. This uniform distribution of the injected materials reduces or eliminates knit-lines and other molding deficiencies which can be caused by temperature differences and/or reaction differences in the injected materials. The RIM process results in generally uniform molecular structure, density and stress distribution as compared to conventional injection molding processes, where failure along the parting line or seam of the mold can occur because the interfacial region is intrinsically different from the remainder of the cover layer and, thus, can be weaker or more stressed.
Furthermore, the RIM process is relatively faster than conventional techniques. In the RIM process, the chemical reaction usually takes place in under 5 minutes, typically in less than two minutes, sometimes in under one minute and, in many cases, in about 30 seconds or less. The demolding time may be 10 minutes or less. The molding process for the conventional methods itself typically takes about 15 minutes. Thus, the overall speed of the RIM process makes it advantageous over the injection and compression molding methods.
The term “demold time” generally refers to the mold release time, which is the time span from the mixing of the components until the earliest possible time at which the part may be removed from the mold. At that time of removal, the part is said to exhibit sufficient “green strength.” The term “reaction time” generally refers to the setting time or curing time, which is the time span from the beginning of mixing until the time at which the product no longer flows. Further description of the terms setting time and mold release time are provided in the “Polyurethane Handbook,” edited by Gunter Oertel, Second Edition, ISBN 1-56990-157-0, herein incorporated by reference.
Polyurethane and/or polyurea polymers are typically made from three reactants: alcohols, amines, and isocyanate-containing compounds. Both alcohols and amines have a reactive hydrogen atom and are generally referred to as “polyols”. They react with the isocyanate-containing compound, which is generally referred to as an “isocyanate.”
Several chemical reactions may occur during polymerization of isocyanate and polyol. Isocyanate groups (—N═C═O) that react with alcohols form a polyurethane, whereas isocyanate groups that react with an amine group form a polyurea. A polyurethane itself may react with an isocyanate to form an allophanate and a polyurea can react with an isocyanate to form a biuret. Because the biuret and allophanate reactions occur on an already-substituted nitrogen atom of the polyurethane or polyurea, these reactions increase cross-linking within the polymer. The stoichiometry of a polyurethane reaction is usually defined as the number of equivalents of active hydrogen groups divided by the number of equivalents of isocyanate groups multiplied by 100. In shorthand, the formula is [—OH or —NH2]/[—NCO]*100. Typical systems utilize a stoichiometry of 95 to 105. A stoichiometry of 95 represents a 5% excess of isocyanate, which ensures that all of the polyol, or soft component, is reacted, providing a fully cured material. A stoichiometry of 105 may be used to obtain a slightly softer material, with the excess polyol acting as a plasticizer. A stoichiometry of 95 to 105 shall be considered balanced. An imbalanced stoichiometry shall be considered to be less than 95 or greater than 105.